Dual SIM telephones are telephones that can hold two SIM cards. Some models are only active on one network at a time and have to be restarted to switch to the other network. They are called DSSS which is the acronym of “Dual SIM single standby”.
Some dual SIM mobile phones can be active on two networks simultaneously in standby mode. In dedicated mode, the phone is only active on the network on which the call is going on. They are called DSDS which is the acronym of “Dual SIM dual standby”.
The Dual SIM telephones are particularly useful in two situations:                For users commuting between two countries, they can have one operator in each country to call with the cheapest rate.        For users having a professional number and a private number.        
The Korean application KR2008058613A disclose a dual mode terminal. The mobile phone uses the two radio stacks reaching signal processing functions and standby functions.
Various other solutions exist for DSDS. Some use two radios, or have two active stacks using the same radio alternatively.
A problem with such solutions is that they have larger power consumption than a single SIM or DSSS phone. This is due to duplication of the signal processing functions and the standby functions. Hence, battery time can be an issue for such phones. As a consequence batteries are typically bigger and/or autonomy is typically shorter.
Thus, one problem of existing solutions is that dual SIM phones are bigger than other phones (due to their larger battery) and/or that battery time is shorter.
Hence, there is a need for methods for dual SIM phones that reduces the power consumption.